04/23/2018

Reconsidering the Role of Aerosols in Deep Convection

A new study shows that observed changes in cloud top height previously attributed to changes in aerosol concentrations in the atmosphere are due to differences in the atmospheric temperature profile.

The Science

All cloud droplets condense around small aerosol particles in the atmosphere known as a condensation nuclei.  Previous studies hypothesized that increasing the number of particles in the atmosphere that can serve as condensation nuclei for cloud droplets may increase the dynamical intensity of deep, moist convection due to increased latent heat release as more droplets form.  The increased dynamical intensity would then cause clouds to grow to increased heights. This study shows that previous studies claiming to observe this effect at the ARM Southern Great Plains (SGP) site in northern Oklahoma are in fact observing an increase in cloud top height because of changes in the atmospheric temperature profile that are correlated with changes in the condensation nuclei concentration. The correlation between thermodynamics and condensation nuclei conditions could be caused by a correlation with prior rainfall in the region, which may lower condensation nuclei concentrations through wet deposition, cool low levels through evaporation, and warm upper levels through condensation.

The Impact

These results suggest that the effect of a four-fold increase in aerosol concentration on convective cloud top heights can be masked by temperature changes of ~1 degree Celsius at the SGP site. This implies that a frequently used method to study aerosol-cloud interactions that consists of analyzing subsets of aerosol data with similar meteorological values is not a valid method for discerning an aerosol indirect effect on convective cloud tops in some conditions. This highlights the need for more careful and detailed accounting of convective meteorological variables in studies analyzing aerosol indirect effects on deep convection.

Summary

This study uses 14 years of observations in warm cloud base, convectively unstable environments to show that surface condensation nuclei (CN) concentration at the ARM SGP site statistically significantly correlates with convective available potential energy (CAPE) and the level of neutral buoyancy (LNB) associated with a buoyancy parcel of air lifted from the level of maximum CAPE. Accounting for correlations between CN concentration and these thermodynamic conditions eliminates the positive correlation between CN concentration and convective cloud top height. This correlation can also be eliminated by simply removing a cloud top temperature mode centered at -10 Celsius, since these clouds likely do not contain ice. The statistically significant correlation between CN concentration and convection-related thermodynamic variables does not appear to be related to the diurnal cycle, seasonality, or synoptic conditions. However, it correlates with regional rainfall accumulation in the 6-hour period prior to convectively unstable, warm-cloud-base conditions at the SGP site.

Principal Investigator(s)

Adam Varble
University of Utah
a.varble@utah.edu

Funding

This study was supported by the Department of Energy (DOE) Atmospheric System Research program under Grant DE-SC0008678.

References

Varble A. “Erroneous attribution of deep convective invigoration to aerosol concentration.” Journal of the Atmospheric Sciences 75(4) 1351-1368 (2018). [DOI:10.1175/JAS-D-17-0217.1]